1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image using toner on a sheet by an electrophotographic method, an electrostatic recording method, a magnetic recording method, or the like.
2. Description of the Related Art
Recently, there is a tendency that the process speed of image formation on a sheet is lowered to reduce noise generated by an image forming apparatus, while a sheet interval is narrowed to achieve conventional throughput. “Sheet interval” refers to the distance from the trailing edge of a preceding sheet to the leading edge of a succeeding sheet when an image is formed successively on a plurality of sheets. However, if the sheet interval is narrowed, erroneous double feed detection and conveyance error are more likely to occur when sheet feed time varies.
Japanese Patent Laid-Open No. 2000-335759 proposes avoidance of double feed of preceding and succeeding sheets by delaying the timing of feeding the succeeding sheet when feed of the preceding sheet is delayed. Japanese Patent Laid-Open No. 5-289453 recites that after a lapse of predetermined period after detection of the leading edge of a sheet by a registration sensor, conveyance of the sheet is temporarily stopped in a state where the leading edge of the sheet is caused to abut a timing roller, and thereafter, the sheet conveyance speed is accelerated. Japanese Patent Laid-Open No. 2000-281247 proposes that a sheet is detected by a sensor arranged upstream of a transfer nip part, and the leading edge positions of the sheet are uniformly aligned without stopping the sheet at a registration roller position.
With the invention of Japanese Patent Laid-Open No. 2000-335759, variation in sheet feeding cannot be disregarded if the sheet interval is narrowed, often resulting in difficulty in achievement of throughput. With the invention of Japanese Patent Laid-Open No. 5-289453, a sheet is temporarily stopped at the registration roller position to make the sheet intervals uniform. However, since the amount of flexure (loop) of a sheet that occurs when the sheet is stopped varies depending on the leading edge position of the sheet, the accuracy of the image formation position on the sheet is likely to decrease. With the invention recited in Japanese Patent Laid-Open No. 2000-281247, a sheet is conveyed at a uniform speed from the registration roller position to the transfer nip part. That is to say, the sheet has to be accelerated to maintain a fixed sheet interval, before the sheet reaches the registration roller position. This requires provision of an acceleration section by extending a conveyance section from a sheet cassette to the registration roller, which leads to an increase in the size of the image forming apparatus.